Internal combustion engine exhaust emissions, and especially diesel engine exhaust emissions, have recently come under scrutiny with the advent of stricter regulations, both in the U.S. and abroad. While diesel engines are known to be more economical to run than spark-ignited engines, diesel engines inherently suffer disadvantages in the area of emissions because fuel is injected during the compression stroke as opposed to during the intake stroke in a spark-ignited engine. As a result, a diesel engine has less time for thorough mixing of the air and fuel before ignition occurs. For this and other reasons, diesel engine exhaust typically contains incompletely burned fuel known as particulate matter, or “soot”.
It is known to use a catalytic particulate filter (CDPF) to trap soot particulates in diesel exhaust gas. During such use, a CDPF progressively loads up with accumulated soot and therefore must be regenerated at operating intervals by burning off the trapped particulates, typically on a fixed schedule and by oxygen and fuel enrichment of the exhaust stream entering the CDPF.
In a typical prior art regeneration, the inlet temperature to the CDPF is targeted to a certain predetermined value, for example, 600° C. The initial process may involve ramping of the CDPF temperature at various rates to prevent excessive combustion of wet soot (soot impregnated with unburned hydrocarbons) which is highly flammable and can cause an uncontrolled combustion that can damage a CDPF. Once the target temperature at the CDPF inlet is reached, that target temperature is maintained until the end of the allowed regeneration (single phase regeneration).
A well-known problem with such a prior art single-phase regeneration scheme is that significant amounts of soot may remain near the front and at the sides of the CDPF. Soot located at the front end tends to burn slowly during regeneration because of relatively low exhaust gas temperature at the CDPF inlet. On the other hand, soot farther along in the CDPF burns faster because of cumulative heat generated upstream. By the end of the designated regeneration period, the front and side portions of a CDPF typically are still covered in soot. This incomplete regeneration reduces the useful size and therefore the overall effectiveness of a CDPF.
What is needed in the art is an improved method for regenerating a CDPF that reduces the amount of soot remaining near the front end and sides of a CDPF after a regeneration procedure.
It is a principal object of the present invention to increase the degree of completeness of regeneration in a CDPF.